Answer to Question #143025 in Zoology for RamC

Question

Why are bony fishes no longer grouped together in a single class “Osteichthyes”? and Why do hagfishes constitute a separate infraphylum?

Answer 1

The endoskeleton of the members of Chondrichthyes is made up of cartilage while the endoskeleton of fish under the class Osteichthyes is composed of bones. ... Among them, 121 species belong to the cartilaginous fish and 1114 species are ray-finned fish.

Class Osteichthyes includes all bony fishes. Like all fishes, Osteichthyes are cold-blooded vertebrates that breathe through gills and use fins for swimming. Bony fishes share several distinguishing features: a skeleton of bone, scales, paired fins, one pair of gill openings, jaws, and paired nostrils.

They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae. Along with lampreys, hagfish are jawless; they are the sister group to jawed vertebrates, and living hagfish remain similar to hagfish from around 300 million years ago.